Activation of syk in an immature B cell line does not require lyn activity.
BKS-2 is an immature B cell lymphoma that undergoes apoptotic cell death when signaled via its surface IgM receptor. To study the signaling components of surface IgM mediated apoptosis in B lymphoma cells, we generated mutants of BKS-2 that were resistant to anti-IgM induced apoptosis. One mutant cell line, 1.B5, did not undergo apoptotic cell death upon treatment with anti-IgM antibodies and also did not exhibit elevation of intracellular Ca2+ in response to cross-linking of surface IgM. This appeared to be due to a defect in protein tyrosine kinase (PTK) activity since fewer proteins were tyrosine phosphorylated in the mutant cells stimulated with anti-IgM when compared to wild type BKS-2. Subsequently, we showed that protein tyrosine kinases lyn and blk were inducibly tyrosine phosphorylated in the wild type BKS-2 but not in 1.B5 mutant cells in response to anti-IgM. Also the kinase activity of lyn was elevated in the wild type but not in mutant cells upon triggering through surface IgM. Furthermore, tyrosine phosphorylation of CD19, a known substrate of lyn, was inducible in anti-IgM stimulated BKS-2 cells but severely reduced in 1.B5 cells. In contrast, kinase activity of another src kinase, blk, was increased on anti-IgM stimulation in both wild type and mutant cells. Surprisingly, syk, a non-src protein tyrosine kinase important for surface IgM mediated signaling, was tyrosine phosphorylated in the lyn deficient mutant cells as well as in the wild type BKS-2 cells. Furthermore, anti-IgM induced increase in kinase activity of syk was similar in the mutant and wild type cells. Thus, in contrast to other studies that propose syk to be a downstream target of src family kinases, syk may act upstream of lyn in immature B cells. Consistent with a functional syk, its target, phospholipase gamma2 (PLC-gamma2) was normally tyrosine phosphorylated in mutant cells.
Muthukkumar
S
S
Department of Microbiology and Immunology and Sanders-Brown Center on Aging, University of Kentucky, Lexington 40536, USA.
Venkataraman
C
C
Woods
T
T
Bondada
S
S
D000818:Animals; D017209:Apoptosis; D001402:B-Lymphocytes / Q000201:enzymology*; D002118:Calcium / Q000378:metabolism; D002455:Cell Division; D002460:Cell Line; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008808:Mice, Inbred CBA; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D011961:Receptors, Fc / Q000378:metabolism; D015398:Signal Transduction; D013499:Surface Properties; D000072377:Syk Kinase; D014443:Tyrosine / Q000378:metabolism; D019061:src-Family Kinases / Q000378:metabolism*
D004792:Enzyme Precursors; D047908:Intracellular Signaling Peptides and Proteins; D011947:Receptors, Antigen, B-Cell; D011961:Receptors, Fc; C035311:immunoglobulin M receptor; D014443:Tyrosine; D011505:Protein-Tyrosine Kinases; D000072377:Syk Kinase; C000606897:Syk protein, mouse; C060902:lyn protein-tyrosine kinase; D019061:src-Family Kinases; D002118:Calcium